phabricator: extract the logic to amend diff properties to a function
This will be needed on a separate code path when dealing with folding revisions.
And since we know that will involve adding multiple local commmits to the diff
properties instead of just one, restructure the logic slightly to allow it.
Differential Revision: https://phab.mercurial-scm.org/D8312
max_width = 79
wrap_comments = true
error_on_line_overflow = true